The conversion of light energy into electrical energy using semiconducting devices has traditionally been performed either with a photovoltaic device or a photoconductive device. Although the physical process for current generation in both devices is the same, photovoltaic devices generally refer to the process of generating useful electrical power from an optical input, for example in a solar panel that converts sunlight into electrical power, while photoconductive devices are used to detect optical signals, for example in a photodetector that converts an optical signal into an electrical signal. Both types of devices require optical photons of sufficient energy to generate an electron-hole pair within the semiconductor, where electrons can be captured by an N-type region in the semiconductor, and holes by a corresponding P-type region. The operational difference between a photovoltaic device and a photoconductive device concerns whether an external reverse bias is applied to the device, or not. For example, in a photovoltaic device, the photogenerated electron and hole are naturally separated by the space-charge region with its built-in electrical field that exists between the N- and P-doped regions, whereas in a photoconductive device, the semiconductor is typically negatively biased, and the photogenerated electron and hole are separated by the electric field of the applied reverse bias. As a result, photovoltaic devices can generate a useful current and a voltage when illuminated, thereby generating electrical power, while a photoconductor device generates a photocurrent, but consumes power in the process since the device is negatively biased by an outside potential.
Any semiconductor material can be made into either a photovoltaic or a photoconductive device. However, some semiconductor materials are better suited to this process than are others. One large division of semiconductors involves whether they have a direct or an indirect energy bandgap. Direct-gap semiconductors convert optical energy into electrical energy more efficiently than do indirect-gap semiconductors. An example of an indirect-gap semiconductor is silicon, a group-IV element, which for practical reasons having to do with manufacturing ease and robustness has become the de-facto semiconductor on which almost all modern integrated circuits are based. An example of a direct bandgap semiconductor is any III-V compound such as GaAs, in which a group-III element (Ga) is combined with an identical number of a group V element (As). Such III-V devices are greatly preferable to silicon for their optoelectronic properties.
The field of optical communications requires the integration of a photodetector with processing electronics. Currently high-performance devices, such as telecommunications transceiver systems, integrate a III-V photodetector (or photoemitter) with silicon processing electronics. However, for purely practical reasons, it is desirable to combine both the photodetector and the integrated electronic circuitry within the same semiconducting substrate, which requires that the photodetector be built using the same manufacturing steps available to the silicon processing electronics such as bulk silicon and Silicon-on-insulator (SOI) processes amongst others. Much research has been centered on designing the best silicon photoreceiver possible given the manufacturing constraints imposed by mass wafer production techniques. Furthermore, this branch of optoelectronics is often centered on reverse-biased photoconductive devices since these devices have a faster optical response time as compared with unbiased photovoltaic detectors. There is far less comparative research on the use of photovoltaic detectors to generate power on standard silicon processing electronics. Most of the research on photovoltaic devices centers on improving standalone photovoltaic cells for solar panel applications, for instance by improving their conversion efficiency or by finding new ways to make standard photovoltaic cells more inexpensive, and not with combining such devices together with other integrated electronic circuits.